This invention relates to solenoidal electric field discharge lamps, and more particularly, to placement of an alloy within the lamp so as to permit the control of mercury vapor pressure within the lamp.
Fluorescent lamps, including solenoidal electric field discharge lamps, operate with the greatest efficiency at a mercury vapor pressure of approximately 7 microns (that is, 7 millitorr). This vapor pressure corresponds to equilibrium with the mercury liquid phase at approximately 40.degree. C. At this mercury vapor pressure, the greatest flux of ultraviolet radiation from the plasma arrives at the phosphor covered wall for a given power input to the positive column discharge of the lamp. However, solenoidal electric field (SEF) discharge lamps are much more compact than conventional tubular fluorescent lamps and thus power densities in SEF lamps are significantly higher. For example, the power input to the discharge plasma divided by the phosphored envelope area is used as a measure of phosphor loading and it is approximately ten times greater in the SEF lamp than in the conventional tubular fluorescent lamp. Thus, the SEF lamp envelope tends to operate at a higher temperature and is typically measured to be approximately 60.degree. C. at its coolest point. The ballast compartment associated with such SEF lamps also runs at approximately the same temperature, that is, approximately 60.degree. C. As a consequence, it is extremely difficult to find a location on the SEF lamp operating at approximately 40.degree. C. for the placement of liquid phase mercury.
The problem of mercury vapor pressure control under varying temperature conditions is solved, at least in part, through the use of various alloys capable of absorbing mercury from its gaseous phase in varying amounts depending upon temperature conditions. Such alloys are known in the fluorescent lamp arts and in particular, certain alloys are described in an article by Bloem et al., titled "Some New Mercury Alloys for Use in Fluorescent Lamps", appearing in Volumne 6, No. 3, of the Journal of the IES, on page 141 in April 1977. The aforementioned article is hereby incorporated herein by reference as background material. Particularly described therein as useful alloys capable of forming amalgams with mercury include a lead-bismuth-tin alloy and a bismuth-indium alloy. The lead-bismuth-tin alloy also possesses the useful property that vapor pressure of mercury is not strongly suppressed at room temperature. Typically a mercury vapor pressure suppression of approximately 50 percent below that over pure mercury results with the use of the lead-bismuth-tin alloy at 20.degree. C., i.e., room temperature. This is a minimal mercury vapor pressure suppression and it permits easier starting of the lamp at room temperature. The use of lead-tin-bismuth alloy also produces a relatively high luminous output over a wide temperature range. However, above a temperature of approximately 90.degree. C., temperature control is lost. This is not, however, a significant problem since the typical SEF lamp operates at a temperature below 90.degree. C.
The placement of these amalgamating alloys in an SEF lamp is, however, a problem for several reasons. For stable, long-term operation the alloy should be placed in a relatively cool, temperature stable location. For purposes of alloy placement, this requirement precludes those regions in the immediate vicinity of a toroidal core employed in SEF lamps, which operate at a higher overall temperature than conventional lamps because of their compactness and the concomitant increase in power density levels. Additionally, alloys which are useable for controlling mercury vapor pressure do not wet well to the glass envelopes employed in SEF lamps even at high temperatures. Thus, placement on the envelope itself, away from the core, is difficult.